Electrostatic focussing devices



Feb. 5, 1963 H. A. c. HoGG ETAL 3,076,909

ELEcTRosTATIc FocussING DEVICES Filed June 3. 1960 United States Patent This invention relates to electrostatic focussing devices. According to the invention, an electrosatic focussing device includes a pair of periodic metal structures disposed with their lengths substantially parallel to each other so as to define between them a space through which an electron beam may travel, elements of one of the periodic structures being respectively disposed opposite at least parts of elements of the other periodic structure, and each structure being so constructed that the elements of the structure are permanently electrically connected together, and two further metal structures extending on either side of and electrically insulated from the pair of periodic structures so that each periodic structure lies between the other periodic structure and one of said further metal structures, the arrangement being such that, upon applying suitable potentials to the periodic metal struc tures and to the further metal structures so that each of the periodic structures is at a positive potential with rcspect to the further metal structures and upon projecting an electron beam in a suitable manner into said space at one end, the beam travels through said space generally in a direction parallel to the lengths of the structures, the beam having an undulatory form such that it alter ately approaches and recedes from each periodic structure.

By a periodic metal structure is meant a series of metallic elements which are spaced apart along a given path (the length or" the structure) and all of which extend in a similar manner transversely to said path. The elements may for example take the form of the teeth of one or more comb-like structures or the rungs of a ladderlike structure.

One arrangement in accordance with the present invention will now be described by way of example with reference to the accompanying drawings, in which:

FIGURE l is a side elevation, shown partly in section, of an electrostatic focussing device in accordance with the invention associated with an electron gun, the section being taken along the longitudinal axis ot the focussing device;

FIGURE 2 is a sectional elevation of the focussing device, the section being taken along the line li-ll of FGURE 4;

FIGURE 3 is a perspective view of part of one of the periodic metal structures;

FIGURE 4 is an enlarged side elevation of a part of the focussing device illustrating its mode of operation; and

FiGURE is a diagrammatic side elevation of a part of a modified forni of the focussing device illustrated in FlGUnFS l to Referring to FIGURES 1 to 4 of the drawings, the electrostatic focussing device includes two periodic structures which are respectively formed by a pair oi parallel rectangular metal plates generally esignated l and 2, each having a length of l0.6 centmetros, a width ot 3.8 centimetres and a thickness of 0.025 centimetro. Th, plates l and Z are spaced apart by a distance of (l2 centimetro and are arranged so that their peripheries arV in register. Each of the plates l and has a series o nine rectangular apertures 3 formed in it, all the apertures 3 being similar in size and having a Width of 0.56 centimetro and a length of 2.1 centmetros. The apertures 3 are spaced apart by d tances equal to their widths with their long parallel and with corresponding ends in register, and the arrangement of the two plates l and Z is such that the elements l formed between the apertures 3 of one plate l or 2 are in register with the elements i formed between the apertures 3 of the other plate fr or l.

A pair of solid parallel metal plates 5 and n having approximately the same overall dimensions as the plates l and 2 are disposed on either side of and parallel to the, plates l and Z, each or" the plates 5 and 6 being disposed 0.127 centimetro from the adjacent apertured plate l or 2. The plates l and Z are spaced apart by means of a number of ceramic washers 7, and are spaced f om, and electrically insulated from, the plates 5 and o by means of a number of further ceramic washers d, the asembly of the four plates Il, 2, 5 and d being held together by means of bolts 9 which pass through the washers and S and through holes in the plates 1i, 2, 5 and 5.

A number of metal pins lo, each 0.15 centimetro in diameter, are secured to the plate 5. The arrangement is such that a pair of pins lil project through each aperture 3 of the adjacent apertured plate l, each pair of pins lil eing symmetrically disposed with respect to the relevant aperture S, beinfr spaced L53 centinietres apart, and projecting across the whole of the gap between the plates l and Z.

The focussing device is associated with an electron gun generally designated ll which is designed to produce a narrow electron beam i2 of approximately rectangular crosssection. The gun li includes an indirectly heated cathode a focussing electrode .ist electrically connect-ed to the cathode i3, and an anode plate l5' having formed in it an aperture lo of rectangular cross-section the longer dimension of which is parallel to the lengths of the apcrtures 3 in the plates l and Z. A molybdenum insert i7 is secured in a recess le (see PGURE 4) formed in that main face of the anode plate l5 remote from the cathode i3, the in crt having formed in it a rectangular slot i9 which also extends in a direction parallel to the lengths of the apertures. 3. in operation, the electron beam l2 passes through the aperture lo and the slot i9, the length of the slot i9 being approximately equal to the width of the beam l2 as it emerges from the gun ll.

rifhe pair of plates l and 2 deliri-e a space through which the electron beam l2 can travel, and are disposed with corresponding ends adjacent the slot i9, these ends being symmetrically disposed with respect to the long edges or the slot The assembly oi the plates l, 2, S and t5 is secured to the anode plate by means of an i.-shaped bracket Ztl, the braclret being secured to the assembly oi the plates l, 2, and d by means of two of the bolts 9; the plate is spaced from and electrically insulated from the pla*e S by means of iurther ceramic Washers The plates l and 2. are electrically connected together and to the anodel plate l5', and the plates 5 and e are also electrically connected together.

The focussing device is also associated with a collector electrode 22 disposed adiacent that end of the assembly of the plates l, 2, and 6 remote from the anode plate l5, the collector electrode 2,?. being secured to the asse1r1- bly of the plates l, il, 5 and o by means oi two of the bolts 9, and being spaced from and electrically insulated from the plate il by means of further ceramic washers 23. The collector electrode includes a U-shaped member Zai the inner surface of the base of which faces the anode plate lf3 and the arms or which are approximately in ter with the plate l and 2.

The combination of the focussing device, the electron gun il and the collector electrode is mounted in an evacuated glass envelope schematically indicated 25', elec- 'rical leads 26 for the anode l5, the cathode i3, the heater aoc/ecos 27 of the cathode i3, the focussing electrode J, the plates E5 and o, and the collector electrode 22 being sealed through the wall ofthe envelope Z5.

in one set of operating conditions, the anode ES is earthed, a negative potential of about SGO volts is applied to the cathode i3 and a negative potential of about 690 volts is applied to the plates 5 and A positive potential of up to 5G volts is applied to the collector electrode 22. The electrostatic focussing device eilectively forms a series of electrostatic lenses spaced apart along the length of the focussing device. The electrostatic held distribution along the general direction of travel of the electron beam l?. between the plates s and 2 is such that there is an overall focussing action upon the' beam l2 in directions perpendicular to the plates il and 2. The er'ect of this rield on the beam l2 is such that the beam l2 has an undulatory form (as illustrated in FIGURE 4), the thickness of the beam l?. (that is to say that dimension of the beam l2 perpendicular to the plates l and Z) varying as it travels along the length of the plates l. and 2, through the axis of the beam l2 is straight and disposed parallel to, and midway between, the longitudinal axes of the plates l and 2. The form of the beam l2 is such that two series of peaks are respectively formed on opposite sides of the beam i2, each series of peaks being disposed in the vicini- -ties of the apertures 3 of the adiacent plate l or 2. Thus, the beam il?. alternately approaches and recedes from each of the plates l and 2 until the beam 12 is incident upon the collector electrode 2.2.

lt is found that, in the arrangement described above, ther is little dispersion of the electrons in the electron beam l2 in directions perpendicular to the plates i and 2, while lateral dispersion of the beam 12 in directions parallel to the plates l. and 2 is largely inhibited by virtue of the two series of pins itl; it is found .that about 80% of the electrons passing through the slot t9 are collected by the collector electrode Z2.

Referring now to FEGURE 5, in an alternative arrangement to that described above the plates l and 2 are displaced slightly with respect to each other so that the elements d of one ot the plates ll and 2 are staggered with respect to those of the other of the plates Il and 2;, half of each element of the plate l being disposed opposite an element of the plate 2 while the remaining halt of the former element 4 is disposed opposite an aperture 3 of the plate 2,. ln this alternative arrangement, the axis of the electron beam l2 follows an undulatory path as the ear l2 travels along the gap between the plates l and 2, the axis of the beam i2 alternately approaching and receding from each of the plates Il and Z. rEhe beam 12 again has two series of peaks formed on opposite sides of it, cach series of peaks being disposed in the vicinities of the apertures 3 o the adjacent plate l or 2. A good transmission ratio is also obtained in this alternative arrangement, with again about 80% of the electrons passing through the slot Il? being collected by the collector electrode 22.

In further alternative arrangements, it may b-e possible for the period of one or the periodic metal structures to be different from that of the other periodic metal structure.

it should be understood that an electrostatic focussing device in accordance with the invention could be incorporated in a travelling wave tube, with at least one of the periodic structures forming at l hst part or a delay device along which an electromagnetic wave may be propso as to interact with an electron beam arranged to travel along the space defined by the periodic structures in a direction generally parallel to the lengths oi the structures. It the arrangern nt were adapted for use as `a back yard wave oscillator, means would be provided for extracting ra 1lio frequency energy from the oscillator at that end of the pair of periodic structures adjacent the electron gun, while if the arrangement were adapted for use as an amplifier, means would be provided for apply- Jin ing a radio frequency signal to one end of the pair ot periodic structures and for taking an output from the other end of the periodic structures.

requirement of a travelling wave tube is that some form or focussing arrancernent must be provided in order to prevent the electron beam from diverging unduly along the length of the tube; for example, in the case of a travelling wave tube employing a helix as a delay device, such focussing is commonly achieved by means of electromagnets arranged so that there is a strong magnetic field directed along the axis ofthe helix. The transmission ratio of each of the focussing devices described above is sufticient for satisfactory operation of a travelling wave tube incorporating such a device, and thus it will be appreciated that the present invention provides a convenient means for focussing an electron beam in a travelling wave tube.

Also, in order to achieve maximum interaction between the electron beam and the electromagnetic wave in a travelling wave tube, the beam should pass as close as possible to the gaps in the periodic structure or structures forming theA delay device since the axial electric lield of the electromagnetic wave is at a maximum in the regions of these gaps. This requirement is especially important in backward wave oscillators since the spaceharmonics of the lield of the electromagnetic wave decay more rapidly away from the periodic structure or structures than the fundamental component of the rield and, in such a device, the electron beam is usually arranged to interact with a reverse space harmonic of the field; by a reverse space harmonic is meant a space harmonic whose phase and group velocities are directed in opposite senses. The achievement of such maximum interaction however involves certain diiiiculties. For example, where the electron beam travels in a rectilinear direction the gaps in the periodic structure or structures are shielded from the electron beam by the elements between which the gaps are formed. Such shielding may be reduced by making each element very thin in a direction perpendicular to the direction of travel of the electron beam, but in such a case the periodic structure or structures will be relatively mechanically weak and will have a low thermal capacity so that distortion or even melting of the elements may occur.

The present invention enables such difficulties to be alleviated by incorporating in a travelling wave tube a focussing device in accordance with the invention in which in operation a set of peaks of the beam are respectively disposed in the vicinities of the gaps in each periodic structure; it will be appreciated that in such a travelling wave tub-e, in operation each peak of the undulatory beam coincides with a region of maximum axial electric field so that there is very good interaction between the electron beam and the electromagnetic Wave without the necessity of making the elements forming the periodic structures very thin.

We claim:

l. An electrostatic focussing device including a pair oi periodic metal structures disposed with their lengths substantially parallel to each other so as to dene between them a space through which an electron beam may travel, elements of one of the periodic structures being respectively disposed opposite at least parts of elements of the other periodic structure, and each structure being so constructed that the elements of the structure are permanently electrically connected together, and two further metal structures extending on either side of and electrically insulated from the pair of periodic structures so that each periodic structure lies between the other periodic structure and one of said further metal structures, the arrangement being such that, upon applying suitable potentials to the periodic metal structures and to the further metal structures so that each of the periodic structures is at a positive potential with respect to the further metal structures and upon projecting an electron beam in a suitable manner into said space at one end, the beam travels through said space generally in a direction parallel to the lengths of the structures, the beam having an undulatory form such that it alternately approaches and recedes from each periodic structure.

2. An electrostatic focussing device according to claim 1, in which the elements of one of the periodic structures are respectively substantially in register with the elements of the other periodic structure.

3. An electrostatic focussing device according to claim 1, in which the elements of one of the periodic structures are staggered with respect to the elements of the other periodic structure so that parts of the elements of one of the periodic structures are respectiveiy disposed opposite parts of the gaps of the other periodic structure.

4. An electrostatic focussing device according to claim 1, in which the periodic structures are each in the form of an elongated metal plate, the gaps of each periodic structure being in the form of slots extending perpendicularly to the length of the relevant plate.

5. An electrostatic focussing device according to claim 4, in which said further metal structures are in the form of metal plates which are respectively spaced by equal amounts from the adjacent periodic structures.

6. An electrostatic focussing device according to claim 5, in which the periodic structures are electrically connected together, and in which said further metal structures are also electrically connected together.

7. An electrostatic focussing device according to claim 1, including means for inhibiting lateral dispersion of the beam as the beam travels through said space.

8. An electrostatic focussing device according to claim 7, in which said means takes the form of two series of metal pins which are electrically connected to one of said further metal structures and which extend through the gaps in the adjacent periodic structure, the two series of pins being spaced apart in a direction perpendicular to the length of each periodic structure and being so arranged that in operation the beam travels between the two series of pins.

References Cited in the ile of this patent UNITED STATES PATENTS 2,857,548 Kompfner et al. Oct. 21, 1958 2,899,597 Kompfner Aug. 1l, 1959 2,951,964 Quate Sept. 6, 1960 2,973,453 Hogg Feb. 28, 1961 

1. AN ELECTROSTATIC FOCUSSING DEVICE INCLUDING A PAIR OF PERIODIC METAL STRUCTURES DISPOSED WITH THEIR LENGTHS SUBSTANTIALLY PARALLEL TO EACH OTHER SO AS TO DEFINE BETWEEN THEM A SPACE THROUGH WHICH AN ELECTRON BEAM MAY TRAVEL, ELEMENTS OF ONE OF THE PERIODIC STRUCTURES BEING RESPECTIVELY DISPOSED OPPOSITE AT LEAST PARTS OF ELEMENTS OF THE OTHER PERIODIC STRUCTURE, AND EACH STRUCTURE BEING SO CONSTRUCTED THAT THE ELEMENTS OF THE STRUCTURE ARE PERMANENTLY ELECTRICALLY CONNECTED TOGETHER, AND TWO FURTHER METAL STRUCTURES EXTENDING ON EITHER SIDE OF AN ELECTRICALLY INSULATED FROM THE PAIR OF PERIODIC STRUCTURES SO THAT EACH PERIODIC STRUCTURE LIES BETWEEN THE OTHER PERIODIC STRUCTURE AND ONE OF SAID FURTHER METAL STRUCTURES, THE ARRANGEMENT BEING SUCH THAT, UPON APPLYING SUITABLE POTENTIALS TO THE PERIODIC METAL STRUCTURES AND TO THE FURTHER METAL STRUCTURES SO THAT EACH OF THE PERIODIC STRUCTURES IS AT A POSITIVE POTENTIAL WITH RESPECT TO THE FURTHER METAL STRUCTURES AND UPON PROJECTING AN ELECTRON BEAM IN A SUITABLE MANNER INTO SAID SPACE AT ONE END, THE BEAM TRAVELS THROUGH SAID SPACE GENERALLY IN A DIRECTION PARALLEL TO THE LENGTHS OF THE STRUCTURES, THE BEAM HAVING AN UNDULATORY FORM SUCH THAT IT ALTERNATELY APPROACHES AND RECEDES FROM EACH PERIODIC STRUCTURE. 